The parts you mentioned are obviously upgraded because the old ones are not performing as well as the rider would like. As for the other parts (bars, seatposts, stem, wheels), these are upgraded often for weight savings. However, as you learn more about yourself, your style of riding, and what's important to you, you will figure out what needs to change on your bike to make it better fit you style.
The first one lets you adjust the height of the seat post on the fly. This would be useful if your ride includes a lot of uphill riding where you'd want a higher seat post, along with steep downhill where a lower seat post is preferred.
The second on lets you adjust the angle of the seat easier that ordinary seat posts.
You can also find suspension seat posts or offset ones.
As you determined there's not a lot of magic in a seat post, stem, handlebar for the most part, unless you're seeking a particular change in dimensions. Weight savings can be minimal and a high cost per gram in any case. I've had a Gravity Dropper for several years and consider it one of the best "upgrades" possible but it somewhat depends on your riding (but can inject new possibilities into whatever riding you do IMHO).
I was having problems with numbness in my left hand when someone suggested handlebars with a greater sweep (the angle the bars bend backward from straight perpendicular). I got a set of Bontrager Race Lite Big Sweep Bars with a 12 degree sweep and my hands now hold the grips at a much more comfortable position with no numbness.
A forum community dedicated to Mountain Bike owners and enthusiasts. Come join the discussion about bike parts, components, deals, performance, modifications, classifieds, trails, troubleshooting, maintenance, and more!